MANUAL SWITCHING OF TELEPRINTER CIRCUITS

An extract from the
Post Office Electric Engineers Journal
Volume 40 - July 1947

The Introduction of Manual Switching Circuits in the Public Telegraph Service
Part 1. The Teleprinter Switching Network

The first part of this article outline the reason for the Introduction of a teleprinter intercommunication system for the Public Telegraph Service, with switching on a manual basis, and describes the switching network provided, and the programme of conversion involved. The second part will deal with the physical and circuit features of the equipment employed.

Introduction
This article is intended to describe not only the Teleprinter Manual Switching (T.M.S.) system employed for the public telegraph 1ervice and the equipment provided, but also to outline the reason for its introduction and to set the whole project in its correct perspective against the background of war, which both fostered and hampered its introduction.

Firstly, it is necessary to stress that but for the war, manual switching would never have been applied to the network, since the system of automatic switching, which had been designed and subjected to satisfactory large-scale field trials prior to the outbreak of hostilities, would have been operative. This automatisation scheme was, however, suspended at the outbreak of war and thus, during the early war years, the telegraph traffic continued to be handled over the existing network of point-to-point circuits, augmented as far as possible to meet the steadily increasing traffic, the trend of which is shown in Fig. 1.
 

FIG. 1 - PUBLIC TELEGRAPH TRAFFIC

With this method of working, a high proportion of the total traffic was dealt with at the Zone Centres but the loss of the telegraph instrument rooms at the C.T.O., Liverpool, Bristol and Birmingham during the 1940/41 blitz, rendered it necessary as a matter of urgency to de-concentrate the telegraph traffic away from the large and vulnerable towns. Under the de-concentration scheme many additional Area Centres were set up, and arranged in "triangles" of three, each triangle having at one or other of the area centres concerned teleprinter point-to-point connections to all other "triangles." The scheme was brought into effect during the summer and autumn of 1941, and although successful in relieving the zone centres of a considerable volume of traffic it resulted in a substantial increase in the number of retransmissions per telegram, with consequent increase both in staff hours and transit time.

The immediate problem of security being resolved, attention was next directed to a means for restoring the quality of service without a corresponding increase in man-power requirements. A reduction in the number of retransmissions by the introduction of switching methods was adjudged to provide the most promising solution, the basis of the switching scheme proposed being the connection of all outlets from a particular "triangle" to all other "triangles" via a switchboard, thus eliminating the need for retransmissions at the intermediate area centre to which these outlets were connected.

At the outset, therefore, the switching scheme was designed primarily as a means of teleprinter communication between area centres only, and some thought was given to the location of the necessary switchboards at appropriate area centres. Study of the equipment requirements showed, however, that more efficient utilisation could be achieved by installing the switching centres at certain selected zone centres, such that, provided the switching equipment was sited in accommodation not more vulnerable than the V.F. equipment, adequate safeguards against dislocation of the service by air attack could be obtained. The decision was taken, therefore, to site the switchboards at selected zone centres of which there were ultimately to be six, and for the purpose of a field trial to install suites of five positions in protected accommodation in the basements of the H.P.O.s at Birmingham and Leeds.

The urgency of the scheme necessitated the avoidance of and delay which the design and provision of special switchboards would have involved, consequently the type of switchboard used by the armed forces on the Defence Teleprinter Network (D.T.N.) was adopted for the field trials with only such modifications as were necessary to meet the differing technical requirements of the Public Service. Although by this means the utmost expedition was achieved in providing and installing the field trial and subsequent equipment, the P.O. Factories Department, which undertook the manufacture of the equipment, was already fully engaged in the supply of D.T.N. switchboard equipment to meet the ever-growing demands of the fighting services, and the authors desire to place on record their wholehearted appreciation of the good services provided both by the staff and works personnel of the Factories Department, under difficult conditions, in meeting these joint requirements, the measure of which is given by the curves in Fig. 2.
 

FIG. 2 - D.T.N. AND T.M.S. SWITCHBOARD PROVISION

The five-position suites at Birmingham and Leeds, comprising the field trial equipment, were brought into service on January 14th, 1944, and from the outset provided a satisfactory service, and afforded a material reduction in retransmissions. Further improvements in the service were expected as more offices were connected, and hence the extension of the switching system to the telegraph network as a whole, by appropriate stages, each of which would contribute its quota to the improvement of the national service, was approved.

It should perhaps be recorded for those readers not familiar with teleprinter switching systems that the circuits terminated at these switchboards are suitable only for the transmission of telegraph traffic and that demands are passed to switchboards by teleprinter messages.

ROUTING OF TELEGRAPH TRAFFIC
Point-to-Point Working
A brief description of the arrangements in force at the outbreak of war, and after de-concentration, are given below to provide an appreciation of the problems affecting the introduction of manual switching.

The number of telegraph offices at which telegrams may be handed in is approximately 14,000, varying in size from small Post Offices to the C.T.O. in London. It follows that it was and is quite impracticable to provide direct circuits between all telegraph offices, or even all large offices, hence for telegraphic communication purposes the country was divided into 11 zones, each of which was divided into areas, and sub-divided into groups. Small telegraph offices known as minor offices were connected to their group centre, which acted as the transmitting office for all such minor office traffic. Similarly, the area centre acted as a transmitting office for all group centres in its area, and the zone centre for its area centres. Direct telegraph circuits were provided interconnecting all zone centres (excepting Belfast, which had connection to only seven zone centres), and in addition, where traffic justified their provision, between zone centres and area centres in other zones, or between area centres. Fig. 3 (a) shows an arrangement of zones and areas illustrating the foregoing principles, and also the trend in the immediate pre-war years to concentrate more traffic on the zone centres, by reduction of the number of area centres and connecting group centres to zone centres direct. The technical arrangements adopted to provide the teleprinter point-to-point circuits concerned have been the subject of previous articles and will not be described here.
 

FIG. 3 - TYPICAL ROUTING OF POINT-TO-POINT TELEPRINTER CIRCUITS

The foregoing covers the purely telegraphic network, but the description would not be complete without reference to the phonogram service and telephone-telegraph system, which together contribute some 40 per cent. of the traffic to be handled by the Inland Service. Telegrams handed in by telephone nay be received either from telephone subscribers or from call offices or kiosks, this service being known as phonograms, or from small Post Offices from which the telegram traffic does not justify the provision of teleprinters, this service being known as telephone-telegrams. The centre at which this telephone traffic is received, and at which special phonogram and T.T. positions are provided and staffed, is known as the "appointed office," and has at least group centre status. Thus onward transmission from the appointed office is nearly always by teleprinter.

The net effect of the point-to-point system of working above described was that a large volume of traffic was concentrated at the zone centre, and although this resulted in efficient utilisation of the main links, an obvious drawback was that a large proportion of the traffic was subject to one or more retransmissions before reaching the objective office. As telegrams have to be circulated for distribution at each transmitting point, and then lined up for onward transmission over the next link, an appreciable aggregate delay was thereby incurred, and additional staff and apparatus costs were clearly involved.

De-concentration Arrangements
As the name implies, the primary objective of the de-concentration scheme was to re-route the telegraph traffic to relieve the vulnerable zone centres of as much transit traffic as possible and thus to safeguard the network against dislocation by air attack. This was achieved by creating many additional area centres, and grouping these in "triangles" of threes, each "triangle" having, at one or other of the three offices concerned, teleprinter outlets to all other triangles. Furthermore, group centres previously connected directly to zone centres were re-parented on outlying area centres wherever practicable. Fig. 3 (b) illustrates the principle of the arrangements n1ade1 and shows. clearly the cost involved in the form of additional retransmissions to secure the necessary safeguards.

Manual Switching-Initial Routing Arrangements
At the outset the Teleprinter Manual Switching scheme was designed primarily as a means of communication between area centres, thus eliminating the retransmission required at intermediate area centres under de-concentration arrangements. Each area centre was to have bothway routes to two, or sometimes three, switching centres, of which there were to be six, located at selected zone centres, and connected each to each by inter-switchboard circuits. Exceptionally, a few large group centres were to be connected to the switching network, generally by a bothway route to the parent switchboard only.

With these arrangements, not more than two switchboards were involved in setting up any connection, and the traffic from area centres requiring two switching operations, particularly in the early stages of the scheme with only two or three switchboards opened, and few group centres connected, would be small. In the ultimate, however, with all switching centres opened, a fair proportion of area centre traffic and a large proportion of group centre traffic would require two switching's, although no real difficulty was expected, as most of the traffic over the D.T.N. network was being similarly routed without difficulty.

Experience during the earlier stages of the scheme showed, however, that the operator output on calls routed through two switchboards was considerably lower than on calls through only one switchboard, the normal tendency towards which was increased somewhat by the effect of the occasional connection of follow-on calls to inter-switchboard circuits which had not been cleared at the distant end, resulting in delays in answering at the second switchboard, as well as misrouted calls. This difficulty was ameliorated by modifying the inter-switchboard circuits to unidirectional working, but to provide a positive solution the design and provision of inter-switchboard line equipment to give full automatic signalling facilities was put in hand. It was also observed, in connection with the use of bothway teleprinter extension circuits from the switchboards, that operators dealing with outgoing calls resented the connection of incoming calls, and therefore tended to hold connections irregularly until ready to originate the next call.

These factors, taken in conjunction with a proposal to extend the number of group centres to be connected to the switching network, led to the design of modified routing arrangements employing unidirectional circuits.
 

FIG. 4 - TYPICAL SWITCHING ARRANGEMENTS, USING UNIDIRECTIONAL CIRCUITS

Manual Switching-Modified Routing Arrangements
The modified scheme for routing the switched traffic over unidirectional circuits, and via only one switchboard, is illustrated in Fig. 4, from which it will be seen that all switchboard routes (with minor exceptions) are divided into incoming and outgoing circuits, and that each office will receive its incoming traffic over the only incoming route provided, namely from the parent switchboard. For outgoing traffic all area centres will have at least one circuit to every switchboard. Thus, from Fig. 4 it will be seen that a call from a to b will be routed a-B-b, whereas a call from b to a will be routed b-A-a. Calls from area centres to group centres will be similarly routed via the home switchboard of the objective centre, e.g. b-B-y, b-A-x. The routing of outgoing traffic from group centres will vary according to its volume ; at the smaller group centres point-to-point circuits will be provided to the home area centre over which all outgoing traffic will be routed for retransmission over the switching network, whereas the larger group centres will have in addition to the point-to-point route, outgoing switching circuits to the home switch board, and to other switchboards where justified by traffic. These arrangements are typified in Fig. 4 by group centres y and x respectively.

The routing of switched public traffic via two switchboards has thus been eliminated, but a small number of unidirectional inter-switchboard circuits have been retained in use, for the routing of service traffic. Later, the use of double switched connections /or a limited amount of public traffic, to meet special cases where on balance a service advantage is realised, is a possibility, and has not been precluded by the technical arrangements made.

The use of a small number of bothway teleprinter extension circuits for special purposes will of course continue, mainly for the provision of service and maintenance facilities, as follows:-

1. Enquiry Circuits. These circuits are provided for dealing with enquiry traffic, and terminate on "EQ" teleprinter positions located near the switchboard positions.
    

2. Speaker Signalling Circuits. With the introduction of manual switching, the speaker circuits provided on control boards in instrument rooms for use by the commercial staff in the setting up of special circuits and so on, have been converted from direct to switching circuits, with consequent economy in the use of V.F. channels. These "Z" circuits appear as bothway circuits on the switchboard, and are terminated on special equipment at the control boards.

Transmission Limits
Allowing for a factor of safety which will ensure satisfactory operation of a teleprinter under average maintenance conditions, the switching circuits have been arranged to ensure that on any switched connection the number of V.F. channels in tandem should not exceed three, when the distortion introduced by any physical section of the connection is negligible.

Teleprinter extension circuits (switchboard to area or group centre teleprinter) are either a combination of physical line and V.F. channels, or are wholly physical. In the former case, the physical section must not exceed the limiting distance for negligible distortion, given in Column 4 of Table 1, whereas in the latter case the limits in Column 3 of Table 1 are applicable, and as the distortion may not then be negligible, not more than 2 V.F. channels in tandem may be employed on connections including such circuits.

The limits quoted in Table 1 have been determined with specific regard to the method of working employed on the T.M.S. network, i.e. 2-wire simplex, using double-current signalling.

TABLE l
T.M.S. Network-Limiting Distances for Physical Extension Circuits

Note 1.-The limits quoted for Twin or Multiple Twin cables apply to side and phantom circuits.

Circuit Provision
The T.M.S. network was necessarily planned to operate on a "no-delay" basis, delay working being incompatible with the objective of improving the over-all grade of service. Considering the groups of unidirectional circuits from switchboards to area or group centres, the circuit provision is determined by the season busy-hour traffic (calculated in T.U.'s), in accordance with the table employed for manually selected "no-delay" telephone junctions. The inter­switchboard circuit provision is similarly determined. The circuit provision for the unidirectional routes from area or group centres to switchboards is determined on a different basis, however, as the telegrams are lined up for onward transmission and manual selection of circuits is not involved. The circuits are provided on an occupied time basis to meet an operator output of 45 telegrams hourly, the average call duration time being 1 minute.

To meet conditions where congestion may exceptionally arise on individual routes outgoing from switchboards, the operating instructions ensure that if the second attempt to secure connection by a forwarding office is ineffective, then the call will be routed on an overflow basis to a teleprinter position at the switching centre concerned for subsequent retransmission. Experience has shown that the amount of overflow traffic, with the above circuit provision, is very small.

CONVERSION PROGRAMME
General
As an ultimate nationwide teleprinter manual switching-network was under consideration at the planning stage, and the prevailing war-time limitations regarding the supply of equipment and availability of installation staff had to be kept in mind, it was envisaged that the switching system should be introduced by a succession of stages, each stage being self-sufficient, and comprising basically the connection of a "triangle" of area centres to the switching network, together with circuits from other area or zone centres working to that triangle. The programme thus provided for ready adjustment to meet unforeseeable contingencies, with a maximum of flexibility in securing the earliest practicable completion date.

The initial programme provided for 15 separate stages to obtain completion of the network, the necessity for the provision and opening to service of the six switching centres arising as follows:-

In addition to the requirements for the manual switchboard installations, the conversion programme also called for the installation of extension teleprinter positions at the various centres connected to the switchboards, and for the necessary provision and rearrangement of V.P. systems to meet the revised line network. To deal in any detail with this latter aspect would perhaps be outside the scope of this article, nevertheless it will be clear that careful planning and close control were necessary to ensure the fulfilment of programme dates while, at the same time. maintaining continuity of service.

A brief review of the progress made with the conversion programme and the results obtained follows. It will be noted that in the event a number of stages have been merged to accelerate the programme, and two stages (5 and 9) deferred for later introduction.
Initial Arrangements.

Stage 1 of the conversion was based on the provision of full switching facilities to the Derby-Leicester­Nottingham triangle of area centres, with switchboard installations (using Switchboards, Teleprinter No. 17) at Birmingham and Leeds. Peterborough, for security reasons, and exceptionally the large group centres of Rugby and Coventry, were also connected as fully switched offices. The remaining group centres in the triangle continued to work to their home area centres over point-to-point teleprinter circuits, as illustrated in the inset to Fig. 5. From this figure, the line network for Stage 1, comprising 200 working circuits, may be discriminated. The outlets from the switching network providing access to the unconverted portion of the existing network are also shown in tabular form in Fig. 5. The extension teleprinter circuits installed at these "partially switched" centres were, of course, of the same design as those provided at the fully switched offices, as described later.

After a period of operation with dummy traffic for the purpose of staff training, the trial network was opened for live traffic on January 14th, 1944. Special traffic returns were taken during the first few months of working, and showed that the average transit time per telegram from the full switching offices had been improved by approximately 20 per cent., and the number of RQs (service messages regarding apparent errors in received telegrams) had been reduced by nearly 50 per cent. The number of retransmissions saved was estimated at 0·8 per telegram. Still further improvements in service were confidently expected as more offices were connected to the network.
 

FIG. 5 - LINE NETWORK FOR INITIAL STAGES

Stage 2 Arrangements
For Stage 2, a third switchboard installation (Manchester) was opened, and the Hull-Grimsby­Middlesbrough area centres, and Darlington group centre added to the switching network as fully switched offices. Additional partially switched offices were also connected to the network, which is shown in Fig. 5. The total of working circuits was increased to 359, and of switchboard positions to 20.

By reason of the different operating procedure employed on the T.M.S network, compared with the D.T.N., it was found during the initial trial that a slight risk was incurred of duplicate connections being established (1 in 1,172 messages), an occurrence which can cause material service difficulties. To preclude such occurrences, a new cord and position circuit design was evolved, and given a field trial on an experimental suite of four positions at Manchester, which comprised the switchboard requirements at this centre for Stage 2. Opportunity was also taken to include a number of refinements in circuit design which bad been developed meantime, as well as of physical details, the resulting switchboard being known as the Switchboard, Teleprinter, No. 17A.

The Stage 2 network was opened for service on December 31st, 1944, with satisfactory results; the automatic "engaged-test" facility incorporated in the Manchester switchboard was proved to be completely effective in preventing duplicate
connections, whatever the cause.

Arrangements for Stages 3 and 4
The combined Stages 3 and 4, opened for service on June 17th, 1945, brought the total of fully-switched offices connected to the switching network to 20, and the total of bothway circuits in use to 546.

Extension of the equipment at the three existing switching centres was necessary, the number of positions being increased to 13, 14 and 9 at Birmingham, Leeds and Manchester respectively, and opportunity was taken to install the extended suite at Birmingham in above-ground accommodation, and thus to improve the staff working conditions.

At the completion of this phase some 28 per cent. of the total public telegraph traffic was being handled by switching means, with a substantially improved grade of service. Nevertheless, traffic observations showed that further improvements could be realised by the introduction of unidirectional working, as previously described.

Arrangements for Stages 6, 7 and 8
This phase of the switching scheme marked the general introduction of the Switchboard, Teleprinter No. 17A, and of conversion of the network to the routing of traffic over unidirectional circuits, as well as the opening of a new switching centre at Bristol. Twelve additional fully-switched offices were connected to the network, and the total of working circuits increased to 867.

The installation work involved the provision of a suite of 20 positions at Leeds, replacing the existing suite of 14 positions in refuge accommodation; of 21 positions at Birmingham, to replace the existing suite of 13 Switchboards, Teleprinter No. 17; of 12 positions at Bristol, and an extension of the Manchester suite by 9 positions (Fig. 6). Also all regions were involved in the provision of additional extension teleprinter positions, as well as the rerouting of V.F. systems to meet the revised line arrangements, illustrated by Fig. 4, and provision of new systems to cater for new offices connected to the network.

An incidental product of the revised method of working was that certain rearrangements of the switchboard face layout to give improved operating facilities were made possible, and were implemented as part of the installation work for this phase, viz. the division of the combined answering and calling jack-field, with 3-panel multiple repetition, into separate incoming and outgoing multiples, with 12-and 4-panel multiple repetition respectively. As a result, the jack-field was simplified from an operating viewpoint, and a material reduction was obtained in the overall height of the multiple field. The self-evident benefit gained by this reduction in height was supplemented, to some extent, by a saving in ineffective time as a result of the reduction in the number of appearances of the calling lamps, and consequent reduction in the number of cases of simultaneous answering by two or more operators.
 

FIG. 6 - TELEPRINTER SWITCHBOARDS AT MANCHESTER

With the introduction of this phase, the provision of equipment at the Birmingham, Leeds and Manchester switching centres catered. for the ultimate requirements, and from a traffic viewpoint, al 24 area centres and a number of group centres in the zones covered by these switching centres had been provided with full switching facilities.

The modified and enlarged network was opened for service on June 23rd, 1946, and functioned satisfactorily from the outset both as regards engineering and traffic facilities.

Stage 10
The fifth switching centre, Glasgow, with six positions, was installed for this stage, and opened for service on January 5th, 1947. The switchboard at the Bristol switching centre was also increased to 18 positions. Belfast and the area centres in the Scottish Region were given full switching facilities, the totality of working circuits being increased to 1,051.

The proportion of the total public telegraph traffic being passed over the switching network at this stage was approximately 40 per cent., or some 70,000 telegrams daily.

Completion Stages
The London zone was the last to be added to the switching network. Because of its magnitude (17 area centres and 54 group centres will be given switching facilities), the conversion was arranged to take place in three phases, of which the first was opened on May 11th, 1947. The remaining two phases will be completed later in the year. During this period, the few residual area centres outside the London zone not yet connected to the switching network will be added as expedient.

The number of switchboard positions installed in the C.T.O., London, to meet the phase I requirements, was 16. For the ultimate requirements 39 positions will be required, with a multiple capacity for 240 incoming circuits and 560 outgoing circuits.

The estimated number of switching circuits in use at the completion of the conversion programme is 1,547, with a total of 134 switchboard positions. The proportion of the public telegraph traffic passing over the switching network is estimated to reach 75 per cent. of the total.
 

An extract from the
Post Office Electric Engineers Journal
Volume 40 - October 1947

The Introduction of Manual Switching Circuits in the Public Telegraph Service
Part 2. The Switching Equipment

The previous part of this article stated the reasons for the introduction of manual switching in the Public Telegraph Service and described the switching network and the programme of conversion. This part describes the switchboards and associated apparatus employed.

Signalling Principles
The standard form of teleprinter circuit used for the public service employs double current signalling on a two-wire. bothway simplex basis. Local copies of transmitted messages are not required and simultaneous communication in both directions of transmission is therefore possible and is, in some circumstances, used. The switching apparatus is designed exclusively for use with circuits of this type, and Fig. 1 is a skeleton diagram showing a typical office-to-office connection via a switchboard.
 

FIG. 1 - SKELETON DIAGRAM OF THROUGH CONNECTION

Following the precedent set by the D.T.N. the line signalling arrangements constitute what may be termed a pulse signalling system. This system is based upon the maintenance of marking (-80V battery) conditions, corresponding to the "rest" condition of the teleprinter, on the line at all times except when engaged in calling, teleprinting, or clearing, each of which operations is effected by transmitting one or more spacing signals.

The signalling conditions in each condition are:-

1. Calling. A series of spacing impulses is transmitted by operating the keyboard of the calling teleprinter. Any combination of signals will suffice, but it has been found convenient, for operating reasons to use the code of the wanted office as, for example, BM BM BM repeated continuously until attention is given by the switchboard operator. The space bar is depressed once after each repetition of the code and the whole transmission should be smooth and continuous.
    

2. Teleprinting. Normal teleprinting conditions, as on point-to-point circuits.
    

3. Clearing. Clearing is initiated by sending a long spacing signal to line and is effective on restoring to the marking condition at the end . of the clearing signal. The clearing signal is applied by a lever key either of normal type or incorporating a mechanically timed release.1 The length of the signal should be at least three seconds to ensure satisfactory clearing under all line conditions, and operators are instructed to hold the key operated (where ordinary lever keys are used) for five seconds. Mechanically timed keys are adjusted to a release period of five seconds.

The Teleprinter Extension Circuit
The Teleprinter No. 3x, which is in general use in the inland telegraph service, is also used on manually switched circuits. This machine has an answer­back attachment which is operated electromagnetically from contacts on the receiving bell crank corresponding to the secondary of letter A. A paper failure alarm device has also been developed for use with teleprinters used for manual switching, as a safeguard to traffic received on positions which are not staffed on a full-time basis.

The answer-back signal is used, primarily, to verify connection to the correct office at the beginning and end of each message and permits messages being received on unattended teleprinters. It is also used in conjunction with the paper failure alarm as a warning to the sending operator that a paper fault has occurred, the circuit being arranged so that answer­back signals are transmitted by the faulty teleprinter so long as the paper fault persists. A calling lamp is provided on each position and is arranged to remain alight after an incoming call until it is reset by a key. A key is also provided to facilitate attention to paper faults and the clearing signal is transmitted by a separate key as already mentioned. The circuit arrangement of an extension office teleprinter is shown in Fig. 2.
 

FIG. 2 - EXTENSION OFFICE TELEPRINTER CIRCUIT

On connection of an incoming call the WRU contacts close due to the caller sending the "Who are you?" signal and the answer-back electromagnet AB1 operates to the charging surge in condenser QA. During the consequent transmission of the answer­back signal, contacts AB1 close and operate the calling relay L which holds to a contact of the reset key. The calling lamp glows until reset by the operator attending to the message.

On outgoing calls, the operator calls the switchboard, using the teleprinter keyboard as already described, and the local calling lamp circuit is not operated. The call is terminated by sending a long spacing signal by the clearing key.

When a message is being received, the paper alarm contacts PF close during each character for approximately 60 milliseconds. Relay P, which is made slow-to-operate by condenser QB, does not respond to these impulses from the PF contacts. In the event of a paper fault, however, the contacts close for at least 150 milliseconds so that relay P operates, causing a relief relay PF and the answer-back electromagnet also to operate. Relay PF locks in the operated position and applies earth to start a flicker pulse generator while also connecting the calling lamp to the flicker pulse. The lamp flashes until attention is given and an audible alarm in the instrument room is operated from the pulse generator.

The answer-back mechanism operates so long as the PF contacts on the teleprinter remain closed and, with a transient fault, the sending operator may proceed with the message when the answer-back signals cease. With a persistent fault, the operator attending to the faulty machine can cut off the alarm conditions by operating the "Paper Alarm Cut-off" key and, the answer-back mechanism having stopped, can advise the caller of the action to be taken. Normally, calls are held and the fault cleared, to minimise risk of duplication of messages.

SWITCHBOARD LINE EQUIPMENT
Provision is made at the switching centre for the following types of line termination:-

• Incoming teleprinter extension.

• Outgoing teleprinter extension.

• Bothway teleprinter extension.

• Incoming inter-switchboard circuit.

• Outgoing inter-switchboard circuit.

Incoming and Outgoing Teleprinter Extension Circuit. A universal form of line equipment, the circuit of which is shown in Fig. 3, is used for these circuits. The relay apparatus is mounted on a 6 ft. 6 in. rack, 1 ft. 8.5in. wide, which caters for 60 lines; a typical line rack is shown in the background of Fig. 4. Each rack is provided with fuse mountings for the 6V power supplies to the calling and F.L.S. lamps, and for the 6V supplies to the relays. Connection strips are provided behind the rack for the F.L.S. strapping for groups of outgoing circuits and for common service cabling. The cabling from the I.D.F. terminates directly upon the relay panels.

Separate multiples are provided on the switchboard for incoming and outgoing circuits, the jumpering on the I.D.F. to the line equipment being varied, as indicated in Fig. 3.

The calling relay L, which is effective only on incoming circuits, is polarised by a metal rectifier so that it responds only to spacing (+80V) signals. It provides a loop across the line wires so that the caller receives his own calling signal until a plug is inserted at the switchboard, when the loop is disconnected by the E relay.

Bothway Extension Circuits
Bothway working is restricted in application to enquiry circuits and to instrument room speaker circuits. The requirements do not warrant the provision of a special rack of line equipment and are met by local modifications to existing incoming and outgoing equipment.
 

FIG. 3 - EXTENSION LINE CIRCUIT, I/C AND O/G, SHOWING CONNECTION OF PLUGGING-UP AND THROUGH CONNECTION JACK

The principal addition to the circuit of Fig. 3, apart from the fact that both calling and F.L.S. lamp circuits are jumpered on the I.D.F., is a slow-releasing relief relay for relay L which steps forward the F.L.S. signal on receipt of a calling signal, the release lag of the relief relay being sufficient to mask any intermittent operation of relay L which may occur during normal calling.
 

FIG. 4 - SWITCHBOARD, TELEPRINTER No. 17B - RACK MOUNTED EQUIPMENT

One bothway circuit occupies the rack space for two unidirectional circuits.

Inter-Switchboard Circuits
Inter-switchboard junction circuits arc routed on V.F. telegraph channels. Calling arrangements are similar to those used on teleprinter extension circuits, and it is normal practice for the calling extension, when extended on a junction circuit by the first switchboard, to resume calling by transmitting the code of the wanted office. The calling relay of the inter-switchboard circuit, as shown in Fig. 5, is also looped between the S and R wires so that the caller again receives the calling signal w1til the distant switchboard operator answers the call.

Although the design of the system provides for through clearing and the supervisory lamps at both switchboards commence to glow virtually simultaneously, it cannot be ensured that both operators will clear down the connection at the same time. It is necessary, therefore, to provide extended engaged facilities so that the F.L.S. at the outgoing end does not indicate a free circuit until the plug has also been withdrawn at the incoming end.

At the outgoing end of the circuit relay G is operated and locks via its own contact when the operator inserts a plug on setting up the call. At the same time, the slow releasing relay F is operated and remains under control of the sleeve relay E. If, when the call is cleared, the incoming operator breaks the connection first, a loop is applied to the S and R lines at the incoming end via the calling relay L. When the outgoing operator then withdraws the plug relay E releases and applies + 80V to the S line via contact F1. The positive signal so transmitted passes round the loop at the incoming end and returns to the line winding of relay G which is connected so that the flux produced neutralises that due to the holding winding and causes the relay to release. The release of relay F restores the normal -80v condition to the line and prevents the calling relay at the incoming end from remaining operated.

If the outgoing operator is the first to clear the connection, the testing signal pulse generated by the combined action of relays E and F cannot return since no loop exists at the incoming end of the circuit. Provision is made at the incoming end, by relays H and J, for a pulse to be sent back to the outgoing equipment when the plug is withdrawn from the jack. Relay G is then released and the F.L.S. marks the circuit as free.

A delayed alarm operates after three minutes if, for any reason, an outgoing equipment is not fully released in this period. Such conditions, apart from abnormal operating delays, are indicative of line faults and require investigation.

The relay equipment is mounted on racks similar to those used for extension line relays (Fig. 4), each rack accommodating 24 incoming and 24 outgoing circuits. The alarm lamps for outgoing circuits are mounted centrally on the relay panels.

FIG. 5 - lNTER-SWITCHBOARD CIRCUIT, I/C AND O/G TERMINATIONS

SWITCHBOARD POSITIONS
It will be seen from Fig. 6 that the switchboard positions follow telephone practice except that, to accommodate the operator's teleprinter, a special arrangement for the key shelf has been incorporated. The teleprinter is mounted on a sliding shelf, which, when pulled forward, facilitates maintenance. The construction is essentially the same as for the floor mounted, multiple type switchboards employed for the D.T.N. in suites ranging from 2 to 25 operators' positions and with multiples from 70-540 lines; hence, flexibility being a major requirement, each position is of unit construction.

The overall dimensions of each position are 2 ft. 5 in. wide, 3 ft. 3 in. deep and 6 ft. 6 in. high.

In addition to the items of equipment visible in Fig. 6, and subsequently described, the switchboards contain a framework at the rear on which are mounted the 75 cord circuit relays and 9 position relays, together with associated rectifiers, condensers and resistors. Chain supported multiple cable bearers are provided at the rear of the jack field, together with a cable rack for supporting miscellaneous cables, and a connection strip to facilitate their termination. Signalling earth, lamp return and rack earth bus-bars are also provided on each position.

Multiple and Face Equipment
Two jack panels, each 14 in. high and 10.5in. wide, are provided on each position. Separate answering and outgoing multiples are employed, the multiple repetition of the former being 12-panel, and of the latter 4-panel. The answering multiple is arranged on the bottom strip of each panel, the calling lamps being below each strip of 20 jacks, and the designation strip (and labels) above. A spacing strip is then inserted between this and the outgoing multiple, in which each strip of 20-line jacks has the associated combined F.L.S. lamp and designation strip above it. The positions shown in Fig. 6 are equipped for 240 incoming and 380 outgoing lines. A miscellaneous jack strip, multipled over every three panels, provides for test facilities.
 

F1G. 6 - FRONT VIEW OF SWITCHBOARD, TELEPRINTER No. 17A, WITH DUMMY END POSITION AND C.T.S.

The white opal lamp mounted over the centre style bar on each position provides a visual "night alarm" indication of an incoming call, or clearing signal on an established connection. The two red opal lamps mounted centrally beneath each jack panel, and designated "engaged" lamps, provide a visual alarm that the operator has over-plugged an already engaged line, and should take down the connection.

Keyshelf
The sloping keyshelf contains 15 cord circuits, each consisting of an answering and calling cord, with associated supervisory lamps, and "PRINT /MONITOR " cord circuit key. An "ANS/CALL" key, which is individual to each position, but common to the 15-cord circuits, is mounted on the left-hand side of the key shelf, and a position "CLEAR" key of press button type is mounted on the right-hand side.

Position Teleprinter
The teleprinter mounted below the keyshelf is the standard B.P.O. Teleprinter No. 3, the electrical connections to which are made by standard plugs and cords on the instrument to a power socket and switch and instrument jack, mounted on the vertical panel at the rear of the sliding shelf.

The teleprinter tape is illuminated by a special lighting fitment mounted on the teleprinter and beneath the teleprinter cover, thus focusing the light on the printing and eliminating any glare to the operator.

Cord and Position Circuits
These circuits, which are illustrated in Fig. 7, are closely related and are best considered together. Special arrangements have been made to prevent interference with established connections due to accidental over-plugging or the duplicate answering of calls in the answering multiple. To ensure this, the tip and ring conductors of all cords are disconnected when the cords are not in use and they remain so until a testing relay in the sleeve circuit has verified that connection is being made to a disengaged line.

Relays SA and SC perform this function for the answering and calling cords respectively and test for a 2500-ohm battery on the sleeve conductor of a free line. If this condition is met, the sleeve relay operates in series with the E relay of the line circuit and applies earth to the sleeve conductor via its low resistance holding winding. The sleeve potential is thus lowered so that no other cord could switch to the engaged line should a plug be inserted elsewhere in the multiple. Any attempt to over-plug is therefore ineffective and harmless.

While the SA and SC relays afford adequate protection, in this way, against the over-plugging of established connections, it is possible for two such relays to operate and hold in parallel if the plugs were inserted into the multiple jacks simultaneously. This cannot readily be avoided in the design of the sleeve relays themselves and, to prevent such a condition from being maintained, common relays are provided in the position circuit which apply a differential current to the high resistance windings of the sleeve relays for a short interval after a plug has been inserted. This current is insufficient to de-flux a sleeve relay which has switched to a free line but ensures the release of any sleeve relay which has operated in parallel with another. In Fig. 7 it will be seen that the sleeve relays derive their operating earth via relay P in the position circuit. Relay P is sufficiently sensitive to operate whenever a plug is inserted, whether the line be free or engaged, and it releases when the sleeve relay operates. Relay P controls relays PR and PZ which, on release of P, apply battery, during the release lag of relay PZ, to the high resistance coils of all operated sleeve relays. Sleeve relays which are released in this manner immediately attempt to re-operate and, owing to the inherent differences in the relay timings of the two cord and position circuits involved, one of them gains sufficient lead after one or two testing cycles to prevent the second sleeve relay from operating.
 

FIG. 7 - CORD AND POSITION CIRCUITS-SWITCHBOARD, TELEPRINTER No. l7A

The unsuccessful connection is indicated to the switchboard operator by an alarm. Relay P remains operated until the plug is withdrawn, since in this case the sleeve relay has not operated and disconnected relay P. Relay DD then operates with a delay of approximately one second, due to the effect of condenser QA. Relay DD causes the operator's teleprinter control relay R to apply spacing conditions to the teleprinter receiver which ''races" and feeds out tape. An alarm lamp is also illuminated beneath each jack panel on the face- of the switchboard. This combination of alarm conditions cannot readily be overlooked by an operator and is maintained until the ineffective plug is withdrawn, thus allowing the position relays to restore to normal. The delay imposed by relay DD is necessary to prevent a premature alarm being given during normal functioning of the sleeve relays.

When the "Print" (KP) or "Monitor" (KM) key of a cord circuit is operated, relay SK operates and extends the tip and ring conductors of both cords via the position circuit. The SK relays arc arranged, in the well-known manner, so that one only can be operated at a time, other cord circuit keys being rendered temporarily ineffective.

Relay H in the position circuit operates in series with the SK relays and removes a holding current from the operator's teleprinter control relay R, leaving this relay free to respond to the conditions applied via the cord circuit tip conductors.

The position "Ans./Call" key (KC) is connected normally in the "Answer" condition so that relay R responds to conditions on the tip conductor of the answering cord. In this position the key also provides for operation of relay A in the position circuit if a cord circuit key is operated to "Print." Relay A in operating applies a loop to the tip and ring conductors of the calling cord so that an extension operator who might be connected would receive her own signals as an indication that the through connection had been interrupted. The tip and ring conductors of the answering cord are connected to the switchboard teleprinter receiving relay and transmitter respectively so that the operator is in teleprinting communication with the party connected via the answering cord.

If the position key KC is operated to the "Call" position relay A in the position circuit releases and relay C operates. The conditions on the cord circuit are thereby reversed, the switchboard operator being in communication via the calling cord and a loop being applied on the answering cord.

To monitor an established connection the cord circuit key KM is operated to "Monitor." Relays SK and H operate as before, but relays A and C remain un-operated. The connection is thus diverted, without interruption, via the position circuit and the operator's teleprinter control relay R is connected in leak on the tip conductor of the answering or calling cord according to the operation of the "Ans./Call" key associated with the position circuit. The operator can, therefore, monitor either direction of transmission without interrupting the call.

A clearing supervisory relay (relays CA and CC) is connected in leak with the tip conductor of each cord, the leak impedance being sufficiently high to have no appreciable effect upon the normal transmission of teleprinter signals. The line coil of each clearing relay is connected in series with a condenser and a metal rectifier. The rectifier is shunted by a resistor so that its effective resistance in the backward or non-conducting direction is controlled within comparatively close limits. When the cord circuit is idle and, apart from short signalling impulses during teleprinting, when it is in use on a connection a negative potential exists on the tip and ring conductors. The clearing Circuit condensers are normally charged, via the forward resistance of the metal rectifiers, to this negative potential. When, during teleprinting, the negative potential on the line conductors is replaced by a corresponding positive one the charge on the condenser tends to become reversed but can only do so via the backward resistance of the metal rectifier and its shunt. The time constant of the circuit is such that the energy which can be stored in the condenser in this way during normal signalling can never reach a value sufficient to operate the relay. The clearing signal of five seconds' duration, however, ensures that the charge on the condenser becomes reversed to the full applied positive potential and, on restoration of the normal negative potential when the clearing key KCL is released, the charge again reverses and produces a rapid surge of current in the relay, via the low forward resistance of the metal rectifier. The clearing relay operates to this surge and holds via its own contact under control of the sleeve relay. The supervisory lamp glows and all relays release when the plugs are withdrawn.
 

FIG. 8 - PLUGGING-UP AND THROUGH CONNECTION PANELS AND CONTROL BOARDS

The potentials available at the cord circuit for operating the clearing relays depend upon the actual voltage of the nominal 80+80 volt supply at the extension offices and upon the total resistance of the sending and receiving lines and instrument terminations. The components of the clearing circuit have been chosen to ensure satisfactory operation on lines up to 1,200 ohms single-wire resistance, using the standard line terminating circuits for teleprinters and V.F. channels and with telegraph supply voltages of 75V or over. At the same time, a safe margin is provided against false operation to teleprinter signals even on lines of zero resistance and with telegraph supply voltages up to 92V. By suitable choice of components a compact design has been achieved in which the size of condenser is reduced to 2µF from the value of 10µF required on previous circuits of this type.

C.T.S., C.S.S. and Dummy Positions
A universal type section, which can be used for cable turning, cable storing or as a dummy position when required for an additional panel to complete a multiple repetition, has been developed for use with switchboards of the type described and with D.T.N. type switchboards. It is shown in use for two of these functions to the left of Fig. 6 and may be utilised without modification, other than in assembly on site. for either end of a suite of positions.

The "Night Alarm" key, by which the night alarm bell is brought into operation, is normally mounted in the panel of the C.T.S.
 

AUXILIARY APPARATUS
Plugging-Up and Through Connection Panel
All lines are routed to the switchboard via the P.U. and T.C. panel on which they appear in conformity with the switchboard multiple layout. The panel is used to provide the following facilities:-

1. By the insertion of a Peg (No. 57), for plugging up faulty circuits.
    

2. By the use of double-ended cords, for establishing " through-connections " between any two lines, thus relieving the switchboard of long duration plug and cord connections.
    

3. By the use of double-ended cords, measuring the send and receive line currents on the test milliammeters located at the top of the panel.

With any of these functions, the lines are disconnected from the switchboard equipment, and the multiple jack positions automatically marked as engaged.

A typical installation is shown to the right of Fig. 8, and the circuit connections in Fig. 3, from which it will be seen that all lines are routed through this panel before connection to the switching equipment.

The position of the P.U. and T.C. panel in the switch room is largely influenced by the routes available for the main cable runway between the switch­board .and the I.D.F.

Common Equipment Rack
Mounted on this rack are the position control relays and associated Bulbs. Resistor No. 11 for 12 switchboard positions. Fig. 4 shows two of these racks in the foreground, from which the location of these items will be clearly seen. The alarm type fuse mountings provide 80+80v supplies to the position relays in the switchboard as well as to the rack mounted control relays; also 6V lamp supply for the position and cord circuit supervisory lamps. The relay panel located centrally mounts the rack fuse alarm relays, although as employed with the initial installations all fuse alarm relays were centralised on these panels. The connection strips at the top of the rack facilitate the connection of cables to the switchboard.

Power Distribution and Fuse Alarm Racks
This rack serves a dual function, primarily for power distribution, but also for the centralised fuse alarms for the switching equipment, and is shown to the right of the common equipment racks in Fig. 4. The cartridge type 80+80V distribution fuses are mounted on the top panel, and the 6V cartridge fuses on the centre panel, together with associated individual alarm type fuses. Alternative sources of power for supplying the 6V A.C. to the switchboard lamps, namely 200-250V A.C. mains, and 230V A.C. standby, are fed to the operative transformer via a mains on/off switch and a triple pole change-over switch. The duplicate transformers are rack mounted, and enclosed in the covers seen at the foot of the rack.

The "No Volt" fuse alarm and exchange urgent and non-urgent alarm relays are panel-mounted at the rear of the rack.

Speaker Signalling Arrangements
Direct speaker circuits equipped with speaker signalling sets have been in use between V.F. telegraph control boards in instrument rooms since the inception of V.F. telegraph working. The advent of the manual switching system made it necessary to provide, in addition to the speakers still required for the ·maintenance of point-to-point circuits, for connections between the control boards and the manual switchboards. In this way the retention of separate speaker channels on a number of routes was avoided with a consequent gain in channels available for traffic. The speaker equipment existing on the control boards was, however, unsuitable for use on the manual switching network and a new design was necessary. Since the use of-two different forms of signalling would have caused confusion it was decided to convert all speaker sets throughout the country to the new design, whether or not direct connection with the switching network was involved. The new speaker signalling set comprises calling, cut-off and clearing relays similar to those used on the Switchboard, Teleprinter No. 17A.

Cord Circuit Tester
For routine testing purposes and as an aid to the clearance of faults, a portable cord circuit tester is provided at each switchboard installation. The tester provides for continuity tests of the plugs and cords and their associated cord circuit wiring and relay contacts, continuity being observed on a milliammeter. "Shake" tests of cords are made while observing the milliammeter which reveals intermittent faults due to worn cords. The tester also provides for lin1iting tests for the clearing relays which must not operate under short line test conditions with clearing signals under 300 milliseconds and must operate under long line test conditions with clearing signals of three seconds duration. Clearing signals for short line tests are timed by a standard telephone dial and can be applied in increasing steps of 100 milliseconds per numeral dialled. The 300 millisecond test signal is produced by dialling 3. Long line tests are made, using a lever key, the operation of which is timed by a stop watch. The tester is used by the traffic staff for day-to-day routine functional tests and by the engineering staff when dealing with faulty or suspected apparatus.

Conclusions
The introduction of manual switching to the Public Telegraph Service has radically changed the method of handling telegraph traffic, and has proved very successful in practice. It must be emphasised, however, that at its inception it was introduced as an emergency war measure, and that at no time has it been out of mind that it is but a temporary method for the more efficient handling of telegraph traffic under the de-concentration conditions resulting from the war, and that, with the introduction of automatic switching, at present under active development, this phase in the metamorphosis of traffic handling will disappear.

Opportunity has been taken to mention the various factors which have influenced the finalised design of the switching equipment now installed at all centres since the experience gained may be of interest and value to others concerned with the manual switching of teleprinter circuits. It should be stressed that without the closest co-operation between the engineering and traffic interests, the experience gained with the initial installations during the war could not have been turned to advantage so speedily as proved to be the case. In this connection, as well as for information contained in this article, the authors wish to acknowledge their indebtedness to Mr. E. W. Cross, of the Telecommunications Department, Inland Telegraph Branch.

In conclusion, a development and conversion programme of the nature described could not have been successfully handled without close co-operation between all parties involved, including Headquarters, Regional and Area staffs. Thanks are also due to colleagues in the Telegraph Branch of the Engineer­in-Chief's office for assistance given in the preparation of this article.

Last revised: August 31, 2023